Connection of Analog and Digial ground

[gogoman]

hello In what instance should a ADC analog ground and digital ground be connected with a ferrite bead. What is the criteria for the bead selection.  the current < 10mA.

thanks

[capt bullshot]

Depends on your design constrains.
Most ADC work best when their analog and digital GND pins are tied to the very same GND plane. So no ferrite bead at all.

[KT88]

In what instance should a ADC analog ground and digital ground be connected with a ferrite bead

Never!
AGND and DGND only denote the internal connections to the IC. Sharing supply currents of analog and digital sections over one bond wire would severely degrade the specs of the converter.
However it makes sense to partition the PCB into an analog and a digital area to avoid interferences between analog and digital signals.